This invention relates to an image forming process, and more particularly, to a process for forming dye images by heating in an alkaline atmosphere.
Photographic processes using silver halides have been most widely used because of their improved photographic properties of sensitivity and gradation control over other photographic processes including electrophotography and diazo process. Recently developed was an advanced technique which can more conveniently and rapidly produce images by substituting a dry treatment by heating or other means for a conventional wet treatment using developing solution in processes for forming images in a photosensitive material based on silver halide.
Heat developable photosensitive materials and heat development process are well known in the art and described in the literature, inter alia, "Fundamentals of Photographic Engineering --Non-Silver Photography--", Corona Publishing K.K., Tokyo, Japan (1982), pages 242-255; "Image Information", April 1978, page 40; Nebletts Handbook of Photography and Reprography, 7th ed., Van Nostrand Reinhold Company, pages 32-33; U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, and 3,457,075; British Patent Nos. 1,131,108 and 1,167,777; and Research Disclosure, June 1978, pages 9-15 (RD-17029).
A number of methods have been proposed for producing color images through heat development. Among others, U.S. Pat. Nos. 3,531,286; 3,761,270; and 4,021,240; Belgian Patent No. 802,519; and Research Disclosure, September 1975, pages 31-32, disclose a process for forming a color image through coupling of an oxidant of a developing agent with a coupler, and a variety of developing agents used therein. Research Disclosure, May 1978, pages 54-58, RD-16966 describes a process of forming an image by incorporating a nitrogen-containing heterocyclic group into a dye to form a silver salt and effecting heat development to release the dye.
A process for forming a positive color image by heat-sensitive silver dye bleaching process is also well known. Useful dye bleaching processes are disclosed in, for example, Research Disclosure, April 1976, pages 30-32, RD- 14433; ibid, December 1976, pages 14-15, RD-15227; and U.S. Pat. No. 4,235,957.
A process for forming a color image using a leuco dye is described in, for example, U.S. Pat. Nos. 3,985,565 and 4,022,617.
These color image forming processes, however, have the drawback that during an extended period of storage, discoloration of color images formed and coloration of white areas occur under the action of coexisting silver halide, silver complex, developing agent and the like. New color image forming processes by heat development which are substantially free of such drawbacks are described in the following Japanese Patent Application Kokai Nos.
______________________________________ 57-179840, 57-186774, 57-198458, 57-207250, 58-58543, 58-79247, 58-116537, 58-149046, 59-48764, 59-65839, 59-71046, 59-87450, 59-88730. ______________________________________
These processes are designed such that a mobile dye is formed or released in proportion or counter-proportion to the reduction of photosensitive silver halide into silver by heat development, and the resultant mobile dye is transferred to a dye-fixing element.
In general, photosensitive materials are preferably developed with the pH of their reaction system maintained high. Incorporating highly alkaline compounds in photosensitive materials gives rise to storage inconvenience. Incorporating such alkaline compounds in image-receiving materials also leads to the accelerated hydrolysis of gelatin and other problems. In a system wherein water is supplied from the outside during transfer or development step, a pH changing compound may be added to the water although there arise problems of storage and handling.
U.S. Pat. No. 3,260,598 discloses an image forming process utilizing the mechanism wherein alkali-releasing agents, a very slightly water-soluble metal hydroxide Z(OH).sub.n and a compound XY are reacted to release hydroxyl ions to increase the pH wherein Z represents a metal atom, X represents a sodium or potassium atom, Y represents a citrate radical, an oxalate radical, a fluorine atom, a ferricyanide radical, a tartrate radical, a sulfite radical, an ethylenedinitrilo tetraacetate radical, a 1,3-diamino-2-propanol tetraacetate radical, a trimethylamine triacetate radical, and other aliphatic nitrogenous polycarboxylate radicals, and n is 2, 3, or 4. This process, however, does not provide fully satisfactory image density. There is a need for further improvement.